1. Field of the Invention.
The present invention relates to a method of fixing and eliminating fluorine and phosphorus in waste water including a fluorophosphate compound.
2. Description of the Related Art.
In recent years, a fluorophosphate compound has been taken into confidence in every direction, and has been increased in quantity consumed. Among others, lithium hexafluorophosphate has been rapidly increased in quantity demanded as an electrolyte for a secondary lithium ion battery. Waste water which arises when a hexafluorophosphate is produced, or when a battery is manufactured, or a waste battery is withdrawn, is necessarily contaminated with fluorophosphoric acid. Since fluorophosphate compounds including PF6xe2x88x92are stable, it is difficult to eliminate fluorine therefrom by merely adding a calcium salt thereto for treatment. In particular, when it is desired to decrease fluorine to an amount of 50 ppm or less in remaining concentration, it is difficult to decrease to such a concentration.
As a method of fixing fluorine from waste water including fluorine so as to eliminate the same, a method which comprises adding a calcium salt thereto so as to produce calcium fluoride to eliminate the same has been commonly used. However, in the case of waste water including a fluorophosphate compound, it is difficult to fix fluorine and/or phosphorus according to this conventional method wherein a calcium salt is added thereto for treatment.
On the other hand, as a method of fixing fluorine in waste water including a fluorophosphate compound, a technique as described in Japanese Patent Kokai (Laid-Open) No. 170380/94 is known.
This technique is the one of fixing fluorine as calcium fluoride, wherein sulfuric acid is added to waste water including fluorophosphate ions so that the concentration of sulfuric acid can be in the range of about 25% to about 35% by weight, and it is heated at a temperature of 200xc2x0 to 80xc2x0 C. for a period of 0.5 to 2 hours, and thereafter, a calcium compound is added thereto.
However, according to this technique, because of using a large amount of sulfuric acid, a large amount of calcium hydroxide is required for neutralizing iron content therein. Furthermore, as result of neutralization, a large amount of calcium sulfate is produced as a by-product, and thus it is necessary to treat the same as industrial wastes.
Besides, the amount thereof to be treated as industrial wastes is immense. For example, when waste water including about 70000 ppm of F is treated in an amount of 1000 kg, a cake thereof is caused as industrial wastes in an amount of 1000 to 1100 kg.
It is an object of the present invention to provide a method of fixing and eliminating fluorine and phosphorus in waste water including a fluorophosphate compound, which makes it possible to reduce the concentrations of fluorine and phosphorus in the waste water including the fluorophosphate compound to a very low level, which has been difficult according to a conventional calcium salt addition method.
It is an object of the present invention to provide a method of fixing and eliminating fluorine and phosphorus in waste water including a fluorophosphate compound, according to which a small amount of an acid is sufficient for use, and the occurrence of industrial waste can be remarkably suppressed.
According to the present invention, a method of fixing and eliminating fluorine and phosphorus in waste water including a fluorophosphate compound is characterized in that al hydrochloric acid is added to the waste water including the fluorophosphate compound so that the concentration of hydrochloric acid in the waste water can be in the range of 2% to 10% by weight: and then the waste water to which hydrochloric acid has been added is heated to a temperature of 80xc2x0 C. to the boiling point of the waste water so as to decompose the fluorophosphate compound into hydrogen fluoride and phosphoric acid, while hydrogen chloride gas in a treating vessel in which the waste water is contained is introduced into a condenser which is provided out of the treating vessel so as to condense and reflux a volatile vapor; and thereafter, a calcium salt is added to the waste water after decomposition so as to fix and eliminate fluorine and phosphorus.
(Hydrochloric Acid)
According to the present invention, first of all, hydrochloric acid is added to waste water including a fluorophosphate compound so that the concentration of hydrochloric acid in the waste water can be in the range of 2% to 10% by weight, so as to acidify the waste water. Incidentally, when hydrochloric acid is added thereto so as to heat and hydrolyze the waste water, it is preferred to agitate the same by using a stirring apparatus.
Fluorophosphate ions are stably in an aqueous solution, and it is said that they are not decomposed even in a boiled state in an aqueous alkaline solution, while are slowly hydrolyzed in an aqueous acid solution.
The present inventor has been devoted to studying the decomposition of fluorophosphate ions. Consequently, it has been found that hydrochloric acid is added thereto, and the mixture is subjected to heat treatment, and thereby fluorophosphoric acid is decomposed into hydrogen fluoride and orthophosphoric acid in a relatively short period of time. With respect to using hydrochloric acid, Japanese Patent Kokai (Laid-Open) No.170380/94 discloses that in order to hydrolyze fluorophosphate ions, even hydrochloric acid can be used; however, when hydrochloric acid is used, the step of heat treatment generates hydrogen chloride gas, and furthermore, the following step of treatment with a calcium compound, particularly the step of treatment with calcium hydroxide, can not fix chlorine, so that hydrochloric acid is drained as waste water, and therefore, it is not practical; thus this official gazette contradicts the practical use of hydrochloric acid.
However, the present inventors have fundamentally reviewed the practical use of hydrochloric acid. Consequently, it has been found that if particular conditions are established, hydrochloric acid can be used, and furthermore it is more practical than sulfuric acid, and besides it is equivalent or superior to sulfuric acid in the ability to decompose fluorophosphoric acid, and thereby the efficiency of fixing and eliminating fluorine and phosphorus can be enhanced.
According to the present invention, hydrochloric acid loadings shall be taken as 2 to 10% by weight; namely, there is a feature in its loadings, wherein this concentration is the one in waste water. Thus acid loadings in the present invention shall be smaller than the ones in the above official gazette. Even such a smaller amount thereof can provide specified effects.
A concentration of less than 2%t by weight is insufficient therefor, wherein it takes too long to carry out hydrolysis. For example, even when waste water is treated therewith at a temperature of 95xc2x0 C. for a period of eight hours, a remaining fluorine concentration is limited to 50 to 100 ppm.
A concentration of about 10% by weight provides a maximum effect thereof. Therefore, even if hydrochloric acid is added thereto so that the concentration thereof can be more than 10% by weight, not only the cost of hydrochloric acid is increased, but also an excessive amount of calcium hydroxide is required to neutralize an excess of hydrochloric acid, which causes an increase in cost. Furthermore, when the concentration thereof is more than 10% by weight, an amount of hydrogen chloride gas which is evaporated during heating is dramatically increased, and thus such a concentration is not preferred.
Incidentally, a concentration of 3 to 6% by weight within the range of 2 to 10% by weight is more preferable.
(Heating Temperature)
According to tho present invention, after adding hydrochloric acid thereto, waste water is heated. A heating temperature is in the range of 80xc2x0 C. to the boiling point of waste water, more preferably 90xc2x0 C. to the boiling point thereof. Incidentally, the boiling point of an aqueous solution including PF6xe2x88x92and hydrochloric acid is on the order of 105xc2x0 C.
Another feature of the present invention lies in this heating temperature; namely, a technique described in the above official gazette in which sulfuric acid is used prescribes that the upper limit of the heating temperature is 80xc2x0 C., and describes that xe2x80x9cwhen the heating temperature it more than 80xc2x0 C., the solution is excessively condensed during treatment, so that it is difficult to carry out the following step of calcium treatmentxe2x80x9d.
In case of hydrochloric acid, the above official gazette enumerates the generation of hydrogen chloride gas as a problem. On the basis of such problems, one of the skilled in the art would usually conceive the idea that the heating temperature should be lowered so as to suppress the generation of hydrogen chloride. However, the present invention is based upon the idea that the heating temperature should be rather elevated so as to accelerate the decomposition of fluorophosphoric acid.
According to the present invention, waste water is heated at a high temperature in the range of 800xc2x0 C. to the boiling point thereof (on the order of 105xc2x0 C.) so as to accelerate the reaction, and thereby fluorine and phosphorus can be fixed and eliminated to a lower concentration, respectively at a shorter period of time. Furthermore, the decomposition of fluorophosphate compound can be more efficiently decomposed.
Incidentally, the above treatment can be carried out even under atmospheric pressure.
(Condensation)
Generally, when a temperature is elevated, a liquid is only too ready to become a gas, and when water, hydrogen chloride or hydrogen fluoride is gasified, so that a volatile amount thereof is increased.
According to the invention, during heating, gases which generate in a treating vessel are introduce into a condenser. When an exhaust vent is provided on the upper side of the treating vessel, and the exhaust vent is connected with the condenser, the heated gases are naturally introduced into the condenser by cooling-condensing action. Additionally, the outlet of the condenser may be provided with an exhauster so as to forcibly introduce generated gases into the condenser. The gases as introduced into the condenser are cooled therein and become a liquid. The components of this liquid comprise hydrogen chloride, hydrogen fluoride and the like which have been volatilized by heating, as well as water. This liquid is recirculated into the treating vessel, and thereby physical matters as introduced into the treating vessel do not escape out of the system. All of toxic materials such as hydrogen chloride and hydrogen fluoride are recovered and eliminated. Furthermore, hydrogen chloride is effectively recycled to decompose fluorophosphoric acid. On the other hand, Japanese Patent Kokai No.170380 points out that xe2x80x9cwhen the heating temperature is more than 80xc2x0 C., the solution is excessively condensed during treatment, so that it is difficult to carry out the following step of calcium treatmentxe2x80x9d: however, according to the present invention, water vapor as evaporated and transpired is condensed and refluxed, so that there occurs no problem of being concentrated to dryness.
(Heat Treating Time)
A heat treating period of time is in the range of 0.5 to 5 hours. When it is less than 0.5 hour, even if a hydrochloric acid concentration and a heating temperature are proper, it is not sufficient for treatment, and the remaining concentration of fluorine in a treated liquid is in the range of 50 to 100 ppm. When a heat treating period of time is more than two hours, the remaining concentration of fluorine in a treated liquid is gradually decreased with the passage of time; however, the concentration falls into an equilibrium state at a range of approximately 20 to 30 ppm. Under proper conditions for treatment, a treatment for a period of 1 to 3 hours is sufficient therefor.
(Calcium Hydroxide)
According to the present invention, after hydrolysis through a heat-treatment, calcium hydroxide is added to waste water so as to convert fluorine and phosphorus into an insoluble salt, respectively, while they are fixed and eliminated. According to this method, hydrogen fluoride and orthophosphoric acid which have been decomposed from fluorophosphate ions are reacted with calcium hydroxide added so as to generate insoluble calcium fluoride and tricalcium phosphate; and/or insoluble hydroxyapatite and fluoroapatite, respectively, which are thereafter precipitated.
A decomposed and treated liquid is quantitatively fed into a neutralizing-treating apparatus for fixing fluorine and phosphorus, wherein the alkalinity of pH=11 to 12 is once made with calcium hydroxide. Hydrogen fluoride and orthophosphoric acid which have been decomposed and generated thereby are fixed as insoluble salts as follows:
2HF+Ca(OH)2xe2x86x92CaF2+2H2O
2H3PO4+3Ca(OH)2xe2x86x92Ca3(PO4)2+6H2O
6H3PO4+10Ca(OH)2xe2x86x92Ca10(PO4)6(OH)2+18H2O
For the fixation and elimination of phosphorus which is an object of the present invention, the object can not be attained by using salt which can generate an acid as a by-product, such as calcium chloride or calcium nitrate. When calcium carbonate is used, a precipitate as produced by reacting calcium carbonate with phosphoric acid is a CaHPO4.2H4O crystal. Since this salt is considerably water-soluble, the remaining concentration of phosphorus in the treated water is in the range of 100 to 200 ppm, which is not sufficient therefor.
When calcium hydroxide is used, the treated liquid is alkaline, and thus the reaction with Ca is further advanced, so that insoluble salts such as higher-order calcium phosphate, hydroxyapatite and/or fluoroapatite are generated and precipitated.
Calcium hydroxide as excessively added is reversely neutralized into pH=6 to 8 according to the usual manner.
A treated liquid obtained in this way is subjected to solid-liquid separation by using a thickener, a separator or the like so as to separate it into an effluent and a solid matter.
On the basis of an official method, the concentrations of fluorine and phosphorus remaining in the treated water are analyzed and the quantity of each of them is determined, wherein waste water is treated under proper treating conditions, fluorine and phosphorus are fixed and eliminated to low levels of 30 ppm or less and 5 ppm or less, respectively.
On the other hand, waste water is treated with calcium hydroxide, hydrochloric acid remaining therein is transformed into calcium chloride according to the following formula:
2HCl+Ca(OH)2xe2x86x92CaCl2+2H2O
Since calcium chloride generated is water-soluble, it remains dissolved in a drainage.
Calcium chloride is often used to eliminate fluorine, with which a degree of eliminating fluorine is larger than the one with calcium hydroxide. This is because calcium chloride is soluble, and thus a ratio of Ca/F during a reaction of eliminating fluorine can be properly set out.
(With Respect to Other Acids)
The kind of an acid with which a fluorophosphate compound can be treated was discussed.
A method of decomposition treatment with perchloric acid is adopted as a conventional method for a pretreatment for analyzing a sample including fluorophosphoric acid and determining the quantity of the same; however, it is necessary to handle this compound with care, and furthermore this compound is expensive, and thus if it is industrially used, there exist problems.
Sulfuric acid, which is used in Japanese Patent Kokai No.170380/94, is also suitable as one of industrial chemicals. However, when sulfuric acid is used in a large amount, in order to neutralize the acid content, the consumption of a large amount of calcium hydroxide is inevitable. Consequently, a large amount of waste is generated, which gives rise to large environmental problems.
Nitric acid also may seem to be effective; however, it should not be used from the viewpoint of the eutrophication of water quality.
Hydrogen fluoride and phosphoric acid also coincide with merely the object of acidifying waste water; however, each of the acids is weak as an acid, and thus satisfactory effects can not be expected.
Hydrochloric acid is a strong acid having a large degree of disassociation, and thus it is expected that hydrochloric acid will have a large effect on acidifying waste water, and furthermore since it is one of general-purpose industrial chemicals, it is available at a relatively moderate cost. The above Japanese Patent Kokai No.170380/94 discloses that hydrochloric acid is volatilized as hydrogen chloride gas during heat-treatment, and thus is disadvantageous; however, if a condenser is provided with a treating vessel, at the same time hydrogen chloride gas is condensed together with generated water vapor, there occurs no problems.
As a treating method of heating and hydrolyzing waste water including fluorophosphoric acid under conditions as mentioned above, both a batch method as can treat fluorophosphoric acid in batch operation and a continuous method as carried out by in series connecting treating vessels with each other can be used.
According to the continuous method, the operation and control of processes can be continuously carried out, and thus this method is remarkably advantageous in efficiency.